An Immutable, Distributed Ledger.
Lets Break it down
Ledger-> collection of financial accounts and transactions (Record of transaction people have made)Immutable-> ledger cannot be changed ever (the record is set in stones)Distributed-> ledger is not controlled by a single entity
BLOCKCHAIN: A distributed network, this means that any given blockchain network is not controlled by a single entity but instead it is run by normal everyday people. So here the ledger is not in the hands of a single company. But is persisted by 1000's of people around there who are all acting ideally and independently. Each person who contributes to the blockchain network is called a NODE. Each node has the exact same copy of the ledger. So the ledger data is hosted and synchronised on the whole network.